Atlantic Ocean Circulation and Climate: The Current View from the Geological

Record

William CurryWoods Hole Oceanographic Institution

AGU Fall MeetingDecember 13, 2006

-30o C

-45o C

Large Scale Ocean Circulation

Modern Atlantic Circulation

The Atlantic Heat PumpThe Atlantic Heat Pump

An Alternative Circulation

Manabe and Stouffer, 1997

Temperature ResponseTemperature Response

• Strong cooling in North Atlantic

• Warming everywhere else

• No net global change

-30o C

-45o C

21,000 yBP

Coring systems

Planktonic foraminifera

Benthic foraminifera

Foraminiferal Calcite

CaCO3

[Ca, Mg, Cd, Zn]

[12C, 13C, 14C]

[16O,18O]

How do you use the fossil chemistry to reconstruct ocean circulation?

• Mapping the spatial patterns of key nutrient tracers (13C) to identify water mass position and geometry

• Using the T-S sensitive tracers (18O) to reconstruct ocean boundary density gradients

NADW

GEOSECS

NADW

Kroopnick (1985)

Foraminiferal chemistry is controlled by the chemistry of the overlying water

Ca13CO3 ~ 13CO2

Core distribution

21,000 year old sample identified at each location

Compiled from many sources

Curry and Oppo (2005) – compiled from many sources

Paleo “GEOSECS” Transect

Curry and Oppo (2005)

Curry and Oppo (2005)

Section at 30o N

Section at 30o S

No data

• Core of North Atlantic Deep Water shoaled from 2500 to 1500 m

• Significantly greater Antarctic Bottom Water flowing north into the North Atlantic basin, influencing water as shallow as 2000m

• Low 13C values found significantly shallower along the eastern margins – more southern ocean water or enhanced remineralization

• The water mass geometry is confirmed by other proxy data like Cd/Ca and 14C

Summary of the Glacial Water Mass Geometry

Temperature and Salinity Characteristics of the Glacial

Deep Waters

Reconstructions of past salinity and 18Ow from pore water profiles in the deep Atlantic

What were the water mass physical properties like?

Pore Water profiles of salinity and 18O of sea water provide clues to the glacial water masses

Adkins et al. (2002)

Antarctic Bottom Water was >1 psu saltier than North Atlantic Deep Water

Antarctic Bottom Water was ~0.5 per mil higher in 18Ow

Both are opposite the modern gradients

Adkins et al. (2002)

Change in Salinity > 1 psu

18O anomaly at 30o S

No data

How did the rate of circulation change?

Ocean margin density gradients and the Geostrophic Method

Large Scale Ocean Circulation

South Atlantic Circulation

Lynch-Stieglitz et al. (2006)

Determining past sea water density

Sea water~ f (T, S)

Foraminiferal CaCO3

18OCaCO3 ~ f (T, 18Owater)

and

18Owater ~ S

Den

sit

y t

Density and 18O

Lynch-Stieglitz et al. (1999)

Density at 30o S

AfricaSouth America

Lynch-Stieglitz et al. (2006)

South AmericaAfrica

Gebbie and Huybers (2006)

The modern circulation can produce these density gradients but to accomplish this……..

the temperature and salinity characteristics of the South Atlantic water masses must have been very different.

Using an Inverse model

Or maybe the northward transport in the upper limb of the overturning circulation really was reduced……

• Consistent with a density gradient reduction across the Florida Straits (Lynch-Stieglitz et al., 1999)

Holocene Glacial Transport

• Consistent with a reduction in the export of 231Pa from the North Atlantic (McManus et al., 2004)

GISP2

Bermuda Rise

H1 YDNADW on

NADW off

0.093

Or maybe the northward transport in the upper limb of the overturning circulation really was reduced……

What can we say about the glacial Atlantic Ocean circulation?

Nutrient proxies depict the changes in geometry and position of the major water masses:

• NADW reduced in total volume, shoaled by about 1000 m and southward penetration significantly reduced

• AABW increased in total volume, influenced the water properties as shallow as 2000 m and penetrated much farther into the North Atlantic

What can we say about the glacial Atlantic Ocean circulation?

Pore water profiles constrain the salinity gradients in the deep Atlantic Ocean:

• AABW was 1 psu higher in salinity than NADW, opposite the gradient observed in the modern ocean

• A large 18O anomaly in the deep Atlantic is associated with this change in hydrography

What can we say about the glacial Atlantic Ocean circulation?

Ocean margin density gradients reflect changes in the mass transport:

• The net northward transport in upper limb of the Atlantic Overturning Circulation was reduced

or

• Thermocline salinities and temperatures were very different on either side of the South Atlantic